On 2/21/2018 2:23 PM, Stefan Beller wrote:

On Mon, Feb 19, 2018 at 10:53 AM, Derrick Stolee [off-list ref] wrote:

quoted

+In order to allow extensions that add extra data to the graph, we organize
+the body into "chunks" and provide a binary lookup table at the beginning
+of the body. The header includes certain values, such as number of chunks,
+hash lengths and types.
+
+All 4-byte numbers are in network order.
+
+HEADER:
+
+  4-byte signature:
+      The signature is: {'C', 'G', 'P', 'H'}
+
+  1-byte version number:
+      Currently, the only valid version is 1.
+
+  1-byte Object Id Version (1 = SHA-1)
+
+  1-byte number (C) of "chunks"
+
+  1-byte (reserved for later use)

What should clients of today do with it?
* ignore it completely [as they have no idea what it is] or
* throw hands up in the air if it is anything other than 0 ?
   [because clearly we will increment the version
    or have new information in a new chunk instead of just sneaking
    in information here?]

They should ignore it completely, which will allow using the value for 
something meaningful later without causing a version change (which we DO 
die() for). A user could downgrade from a version that uses this byte 
for something meaningful and not require a new commit-graph file.

The "commit-graph read" subcommand does output this byte, so we can 
verify that the "write" subcommand places a 0 in this position.

quoted

+CHUNK LOOKUP:
+
+  (C + 1) * 12 bytes listing the table of contents for the chunks:
+      First 4 bytes describe chunk id. Value 0 is a terminating label.
+      Other 8 bytes provide offset in current file for chunk to start.

offset [in bytes? I could imagine having a larger granularity here,
because chunks don't sound small.]

It is good to specify "offset in bytes".

quoted

+      (Chunks are ordered contiguously in the file, so you can infer
+      the length using the next chunk position if necessary.)
+
+  The remaining data in the body is described one chunk at a time, and
+  these chunks may be given in any order. Chunks are required unless
+  otherwise specified.
+
+CHUNK DATA:
+
+  OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes)
+      The ith entry, F[i], stores the number of OIDs with first
+      byte at most i. Thus F[255] stores the total
+      number of commits (N).

[ so in small repos, where there are fewer than 256 objects,
F[i] == F[i+1], for all i'th where there is no object starting with i byte]

Correct. I'm not sure this additional information is valuable for the 
document, though.

quoted

+  OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes)
+      The OIDs for all commits in the graph, sorted in ascending order.
+
+  Commit Data (ID: {'C', 'G', 'E', 'T' }) (N * (H + 16) bytes)
+    * The first H bytes are for the OID of the root tree.
+    * The next 8 bytes are for the int-ids of the first two parents
+      of the ith commit. Stores value 0xffffffff if no parent in that
+      position. If there are more than two parents, the second value
+      has its most-significant bit on and the other bits store an array
+      position into the Large Edge List chunk.
+    * The next 8 bytes store the generation number of the commit and
+      the commit time in seconds since EPOCH. The generation number
+      uses the higher 30 bits of the first 4 bytes, while the commit
+      time uses the 32 bits of the second 4 bytes, along with the lowest
+      2 bits of the lowest byte, storing the 33rd and 34th bit of the
+      commit time.
+
+  Large Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional]
+      This list of 4-byte values store the second through nth parents for
+      all octopus merges. The second parent value in the commit data stores
+      an array position within this list along with the most-significant bit
+      on. Starting at that array position, iterate through this list of int-ids
+      for the parents until reaching a value with the most-significant bit on.
+      The other bits correspond to the int-id of the last parent.
+
+TRAILER:
+
+       H-byte HASH-checksum of all of the above.
+
--
2.7.4

Makes sense so far, I'll read on.
I agree with Junio, that I could read this documentation without
the urge to point out nits. :)

Thanks,
Stefan